Gaussian lenses are conventionally used as a standard bright lens for photography and various types of these lenses have been developed for various purposes, such as for realizing a large aperture, compactness, high performance, and low cost. For example, Gaussian lenses disclosed in Japanese Laid Open Patent Applications Nos. H6-337348 and H11-183792 are known. These lenses include a joined lens within one of front and rear lens groups that are positioned on either side of a stop.
The two joined surfaces of a joined lens are very critical in considering aberrations because they tend to have a large curvature to ensure a small Petzval""s sum and achromatism. In the conventional Gaussian lenses described above, spacing lens surfaces (i.e., so that they are in air) is useful for increasing the parameters available for control of aberrations. More specifically, spacing the lens surfaces allows the spherical aberration and astigmatism to be more satisfactorily corrected in order to improve performance of the lens. However, spaced surfaces in air tend to produce large aberrations. This imposes a need for high precision in assembling the lens components and this may impair the suitability of the lens design for manufacturing. Thus, some lens element surfaces are usually joined.
Producing a Gaussian lens which meets requirements such as being compact yet having a large aperture, high performance and low cost is usually accomplished using a glass material having a high index of refraction. Such a glass material, unfortunately, is difficult to work (in terms of shaping a lens component), and the lens elements formed of such a material often require a high precision of assembly. Therefore, for standard camera lenses that are to be mass produced, suitability of the lens design for manufacturing and assembly is often emphasized with the result that lens performance may suffer.
As Gaussian lenses without joined surfaces, Japanese Patent No. H8-33512 and Japanese Laid Open Patent Application No. H8-220424 describe Gaussian lenses wherein all the lens elements are separated by air. However, these Gaussian lenses, formed of front and rear lens groups with each lens group consisting of three lens elements, have rather narrow image angles.
The present invention relates to a high performance Gaussian lens for photography which is suitable for use with a single lens reflex camera. The object of the present invention is to provide a Gaussian lens that forms a flat image surface for the entire range of image angles. This is accomplished by using lens element surfaces which are all spaced apart in air, while ensuring that the Gaussian lens may be manufactured using conventional methods at a reasonable cost.